Daniel is 4 times as old as Jessica and is also 21 years older than Jessica. How old is Daniel?
Explanation: We can use the given information to write down two equations that describe the ages of Daniel and Jessica. Let Daniel's current age be $d$ and Jessica's current age be $j$ $d = 4j$ $d = j + 21$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $d$ is to solve the second equation for $j$ and substitute that value into the first equation. Solving our second equation for $j$ , we get: $j = d - 21$ . Substituting this into our first equation, we get the equation: $d = 4$ $(d - 21)$ which combines the information about $d$ from both of our original equations. Simplifying the right side of this equation, we get: $d = 4d - 84$ Solving for $d$ , we get: $3 d = 84$ $d = 28$.